Many touch screens, such as those used in mobile phones and tablets, are equipped with proximity detectors. These detectors, often using infrared transmitters, are designed to detect simple gestures, such as the approach of an object. This detection is used, for example, to disable the touch screen function during a call when the phone is near the ear of the user.
Infrared sensors typically use the brightness reflected by the target object. Thus, a distance evaluated using an infrared sensor is a rough estimate, since it depends on the reflectance and the tilting of the target object.
Patent application US20130175435 discloses a proximity detection technique using a photon time of flight sensor.
FIG. 1 schematically illustrates an implementation of a time of flight sensor as described in the aforementioned patent application. The sensor includes, on a printed circuit board 8, an infrared radiation source 10 emitting photons in a cone 12. A photon detector 14 is arranged on the printed circuit board close to the transmitter 10 for receiving photons reflected from a target object placed in the cone 16. The detector 14 is based on so-called single photon avalanche diodes (SPAD).
A control circuit, not shown, energizes the transmitter 10 with short duration pulses and observes the signal from the detector 14 to determine the elapsed time between each pulse and the return of a corresponding burst of photons on the detector 14. The circuit thus measures the time of flight of the photons along a path 18 going from the transmitter 10 to the object 16 and returning to the detector 14. The time of flight is proportional to the distance between the object and the detector, and does not depend on the intensity of the received photon flux, which varies depending on the reflectance of the object.
A time of flight sensor of the type of FIG. 1 thus makes it possible to accurately determine the distance of a target object relative to the sensor without being influenced by the reflectance of the object or parasitic phenomena that alter the intensity of the photon flux received by the detector, such as the interposition of a tinted glass.